The field of medicament delivery devices is becoming increasingly important, and in particular injection devices for self-administration of medicament is becoming more and more popular.
However, there are a few aspects that affect the possibilities of successfully marketing an injection device, satisfying requirements from both injector manufacturers, drug manufacturers and of course the users of the devices.
Regarding manufacturing and sales of injection devices one requirement is that it must not be too expensive to manufacture, and the cost is very much depending on the complexity of the device regarding functions because added functions mean additional components that have to be designed so that they interact flawlessly in order to ascertain proper function of the device. Regarding the sales and end users, the device should preferably be rather discrete at the same time as it should be easy to use, almost intuitive at the same time as it should be safe enough to avoid unintentional firing of the device.
Another aspect regarding medicament delivery devices is how to expel the medicament. Often the medicament is arranged in a compartment having a movable wall, like a stopper, and an orifice in the opposite end, to which orifice a needle is attached, like a syringe or cartridge.
Often a spring or the like force means is connected to a plunger rod, in turn acting on the stopper for expelling the medicament. The spring is in most applications a helically wound compression spring, which is a well tried and economical drive member for an injection device. However, it has a few drawbacks that can affect the function of the device. One drawback is that the spring force is highest at the beginning of the movement of the plunger rod, and that the spring force decreases linearly with the extension of the spring, until it reaches almost zero force when almost fully extended. If the medicament has a rather high viscosity, it requires rather high forces to push it through the narrow passage of the needle, where the forces have to be available during the whole stroke of the plunger rod. With a compression spring, it has to be “over-dimensioned” in order to handle the whole stroke.
Another drawback with the compression spring can be that the force is highest at the beginning of the stroke, and if a sudden high force is applied to the cartridge or syringe often mad of glass, it may break, causing a serious malfunction of the device.
Attempts have been made to arrange force members providing more constant force during the stroke, for example leaf springs. However, they tend to require more space than spiral springs, thus making the devices larger.
Attempts have also been made to arrange force members providing more constant force during the stroke, for example gas springs as in US2004/0254525, US2002007149 and U.S. Pat. No. 5,919,159. However, these members have been only provided in needle-less delivery devices which require high driving pressure without damping for driving the drug through a nozzle into the outermost layer of the skin. Since the thickness and penetrability of the skin varies from patient to patient, it is complicated to regulate the level of pressure to drive the drug correctly through the outermost layer of the skin and many patients experience said needle less injections as painfully.